Mg Alloys Containing  Misch Metal Manufacturing Method of Wrought Mg Alloys Containing Misch Metal, and Wrought Mg Alloys Thereby

ABSTRACT

There are provided a magnesium alloy with a misch metal, a method of producing a wrought magnesium alloy with a misch metal, and a wrought magnesium alloy produced thereby, in which a great deal of misch metal is added to magnesium, and thus refractory eutectic phases or multi-phases are formed into a stable network structure or a stable dispersed phase, thereby inhibiting deformation of a magnesium matrix at a high temperature to maintain a high strength. The magnesium alloy with the misch metal has the formula of Mg 100-x-y-g A x B y C z , where A is zinc (Zn) or aluminum (Al); B is the misch metal; C is at least one element selected from the group consisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn), yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at %≦z≦2 at %, respectively.

DESCRIPTION

1. Technical Field

The present invention relates to a magnesium alloy with a misch metal,in which a great deal of misch metal is added to magnesium, therebyhaving a network structure or a dispersed phase which is stable at ahigh temperature and thus exhibiting excellent mechanical properties.Further, the present invention relates to a method of producing awrought magnesium alloy which granulates solidification structures, i.e.secondary phases or multi-phases, by means of hot extrusion and hotrolling, and ultra-refines grains of a matrix, and a wrought magnesiumalloy produced by the method.

2. Background Art

Nowadays, as environmental and saveenergy problems attract a lot ofattention all over the world, it is absolutely required to make partslighter in weight. There are stronger and stronger requests that anenvironmental pollution problem resulting from carbon dioxide generatedduring transportation by road, aviation, and rail should be solved, andthat parts or end-products should be made lighter in order to save atransportation fuel. Under this situation, a magnesium alloy suggests amost efficient possibility of making the products lighter, because ithas the lowest density among commercial alloys, namely ⅔, and ⅕ times aslow density as an aluminum alloy, and a ferrous alloy, respectively. Inaddition, the magnesium alloy has excellent specific strength, rigidity,vibration absorptivity, machinability, dimension stability, andelectromagnetic wave shielding effects, so that it is widely used assheathings of electronic/telecommunication products such as mobilecommunication equipment and laptop computers.

In general, the magnesium alloy for a high-temperature structure isclassified into two types: a casting magnesium alloy used without heattreatment, and a sand casting magnesium alloy in which high-temperatureproperties are improved by precipitating a secondary phase in a matrix.

In the casting magnesium alloy, because a molten metal frequentlygenerates an eddy when passing through the gate of a metal mold to entera cavity, its product contains a number of blowholes. These remainingblowholes results in generating a blister on a surface of the productduring heat treatment including solution heat treatment in the future,the product is not typically subjected to the heat treatment.Accordingly, an AZ91 alloy, a Mg—Al alloy, which is widely used as thecasting magnesium alloy at the present time, is low in high-temperatureproperties, especially creep resistance. Hence, the AZ91 alloy has adifficulty in being applied to parts exposed to a high temperature (150°C. or more) such as a transmission case of an automobile. This isbecause, when aluminum is added to magnesium, room-temperature strengthand fluidity of the molten metal are improved, but a phase of Mg₁₇Al₁₂is formed to deteriorate the creep resistance property at a hightemperature. In order to overcome this drawback, either addition ofearth-rare elements or addition of calcium (Ca), silicon (Si), strontium(Sr), etc. as disclosed in U.S. Pat. No. 6,264,763 is carried out.However, so far, there is a limit to utility in the aspects ofproductivity, mechanical properties including the high-temperature creepproperty and corrosion resistance, and costs.

In the sand casting magnesium alloy, the secondary phase is precipitatedin the matrix by the heat treatment, and thereby high-temperaturestrength and heat-resistant property are improved. Thus, it is possibleto obtain a relatively sound casting. For this purpose, additionelements should has a great solubility change in a magnesium matrixaccording to temperature, and maintain soubility at a temperature of200° C. or more, which is mainly used. As main addition elements of thesand casting magnesium alloy, silver (Ag), thorium (Th), yttrium (Y),neodymium (Nd), scandium (Sc), etc. are used, each of which is tooexpensive or contains a radioactive substance. Thus, these elements havebeen restrictively used for the case of giving a greater weight onperformance than a cost.

Meanwhile, conventionally, there is a technical problem on formation ofthe magnesium alloy. In principle, the magnesium alloy has a hexagonalclose packed structure and restrictions of a slip system required forplastic working. For this reason, it is very difficult to form a productat a room temperature. Hence, the product should be formed through hotworking.

In this manner, in order to develop intermediate products andend-products of the magnesium alloy, it is absolutely necessary toimprove formability. It is the most effective method that is to refine acrystal structure of the magnesium alloy to improve an elongation. Inaddition, the magnesium alloy of the fine grain structure should besubstantially produced by an industrial method. The demand for amagnesium alloy plate is gradually increased. However, the magnesiumalloy plate having a required fine grain structure is not efficientlyproduced using a currently commercialized magnesium alloy.

In the case of an existing AZ31 alloy, a reduction in thickness duringrolling should be increased in order to refine the grain. In this case,the reduction in thickness is restricted due to serious cracking, andthus the grain refinement is restricted. In other words, this solidsolution alloy is restricted in a source capable of generatingrecrystallization in its interior, and thus has a limit to the grainrefinement.

DISCLOSURE Technical Problem

It is an objective of the present invention to provide a magnesium alloywith a misch metal, in which a great deal of misch metal is added tomagnesium, and thus refractory eutectic phases or multi-phases areformed into a stable network structure or a stable dispersed phase,thereby inhibiting deformation of a magnesium matrix at a hightemperature, and which other elements are additionally added, and thusprecipitation/solid-solution is strengthened in a matrix structure orthe network structure is strengthened, thereby having excellentmechanical properties in which high strength is maintained at a hightemperature.

It is another objective of the present invention to provide a method ofproducing a wrought magnesium alloy, in which a secondary phase ormultiphase magnesium alloy to which a misch metal is added granulatessolidification structures, i.e. secondary phases or multi-phases, bymeans of hot extrusion and hot rolling, and recrystallizes a matrix,thereby refining grains.

It is another objective of the present invention to provide a wroughtmagnesium alloy, which has a fine grain structure, exhibits mechanicalproperties such as high strength and high toughness in aroom-temperature area in which the alloy is substantially used, having agood elongation at a temperature at which formation is substantiallycarried out, and thus is improved in formability.

Technical Solution

To accomplish these objectives, a magnesium alloy with a misch metalaccording to the present invention has the formula expressed byMg_(100-x-y-z)A_(x)B_(y)C_(z), where A is zinc (Zn) or aluminum (Al); Bis the misch metal; C is at least one element selected from the groupconsisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn),yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and x, y andz are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at%≦z≦2 at %, respectively.

Further, the misch metal may be a didymium-based misch metal or acerium-based misch metal.

Here, the didymium-based misch metal may be a rare earth alloycomposition including neodymium (Nd) and praseodymium (Pr).

Also, the cerium-based misch metal may contain 45 wt %≦Ce≦65 wt %, 20 wt%≦La≦30 wt %, 5 wt %≦Nd≦15 wt %, and 0 wt %≦Pr≦10 wt %.

Furthermore, the magnesium alloy may further comprise calcium of 2 at %or less.

A method of producing a wrought magnesium alloy with a misch metalaccording to the present invention comprises the steps of:fusion-casting a magnesium alloy composition having the formula ofMg_(100-x-y-z)A_(x)B_(y)C_(z), where A is zinc (Zn) or aluminum (Al); Bis the misch metal; C is at least one element selected from the groupconsisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn),yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr);

and x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at%, and 0 at %≦z≦2 at %, respectively; and hot-extruding the cast, andrefining grains through granulation and dispersion of other phases thanmagnesium in the cast, and recrystallization of a matrix.

Here, the method may further comprise a step of hot-rolling thehot-extruded product to form a plate.

Further, the hot-extruding step may be performed under the extrusionconditions of a temperature range from 350° C. to 450° C., and a ratioof reduction in section of 5˜80:1.

Also, the hot-rolling step may be performed under the rolling conditionsof a temperature range from 350° C. to 500° C., and a percentage ofsingle reduction in thickness from 25% to 50%.

Further, a wrought magnesium alloy with a misch metal is produced by thesteps of: fusion-casting a composition having the formula ofMg_(100-x-y-z)A_(x)B_(y)C_(z), where A is zinc (Zn) or aluminum (Al); Bis the misch metal; C is at least one element selected from the groupconsisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn),yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and x, y andz are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at%≦z≦2 at %, respectively; hot-extruding the cast, and refining grainsthrough granulation and dispersion of other phases than magnesium in thecast, and recrystallization of a matrix; and hot-rolling thehot-extruded product to from a wrought product.

Here, the other phases than magnesium may have a size of 20 μm or less.

Further, the other phases than magnesium may be contained from asolid-solution limit to a eutectic point or a hyper-eutectic area.

ADVANTAGEOUS EFFECTS

As described above, in the magnesium alloy with the misch metalaccording to the present invention, the misch metal is added, and thusrefractory eutectic phases or multi-phases are formed into a stablenetwork structure or a stable dispersed phase, thereby inhibitingdeformation of a magnesium matrix at a high temperature. Further, otherelements are additionally added, and thus precipitation/solid-solutionis strengthened in a matrix structure or the network structure isstrengthened, thereby having excellent mechanical properties in which ahigh strength is maintained at a high temperature.

Further, in the method of producing the wrought magnesium alloyaccording to the present invention, a secondary-phase or multiphasemagnesium alloy to which the misch metal is added is recrystallized bythe hot extrusion and hot rolling, and the grains are refined.

In addition, the wrought magnesium alloy with the misch metal accordingto the present invention has a fine grain structure, and thus exhibitsmechanical properties such as a high strength and a high toughness in aroom-temperature area in which the alloy is substantially used. Further,the wrought magnesium alloy has a good elongation at a temperature atwhich formation is substantially carried out, and thus is improved informability.

In this manner, the magnesium alloy with the misch metal havingexcellent mechanical properties according to the present inventionsatisfies the requirements of high strength and heat resistance whichare required for power transmission parts of a vehicle.

Further, by adding calcium (Ca) to the magnesium alloy with the mischmetal according to the present invention, fusion in air and casting arepossible, so that it is possible to promote saving of production costs.

Also, the magnesium alloy with the misch metal according to the presentinvention exhibits a high-temperature strength better than aheat-resistant magnesium alloy produced by existing heat treatment, sothat it can be applied to parts for the vehicle and aircraft.

In addition, the magnesium alloy with the misch metal according to thepresent invention exhibits relatively better corrosion resistance than apreviously commercialized heat-resistant magnesium alloy, so that it isused for lightweight parts capable of enduring severe conditions such ashigh temperature and corrosion.

Meanwhile, in the method of producing the wrought magnesium alloyaccording to the present invention, a magnesium alloy plate containing agreat deal of ultra fine particles can be produced, and the producedplate has fine grains and very excellent formability. Accordingly, thewrought magnesium alloy of the present invention can make it lighterroad, aviation, and rail transportations, and be widely used assheathings of electronic/telecommunication products such as mobilecommunication equipment and laptop computers.

DESCRIPTION OF DRAWINGS

FIG. 1 is a scanning electron microscope photograph showing a networkstructure of an alpha magnesium structure and a Mg₁₂Ce phase in Alloy 3of Table 1;

FIG. 2 is a photograph of a molten metal, in which 2 wt % calcium (Ca)is added to Alloy 9 of Table 1 and fused in air;

FIG. 3 is a photograph showing a cast structure of an magnesium alloyaround a eutectic point, according to a fifth embodiment;

FIG. 4 is a microstructure photograph showing a wrought product obtainedby performing hot extrusion on a magnesium alloy at a temperature of450° C., an extrusion speed of 2 mm/sec, a ratio of reduction in sectionof 6:1, according to a fifth embodiment;

FIG. 5 is a microstructure photograph showing a wrought product formedby rolling a magnesium alloy under a roll temperature of 100° C. with asingle reduction in thickness of 40% at a temperature of a test piece of400° C., according to a fifth embodiment;

FIG. 6 is a photograph showing wrought products that is prepared on theconditions of hot extrusion and rolling performed in a fifth embodiment,and subjected to a high-temperature tension test with strains 1×10⁻³s⁻¹, 1×10⁻² s⁻¹, 1×10⁻¹ s⁻, and 1×10⁻⁰ s⁻¹ at a temperature of 500° C.,together with elongations of the wrought products;

FIG. 7 is a microstructure photograph showing a wrought product obtainedby performing hot extrusion on a magnesium alloy at a temperature of450° C., an extrusion speed of 2 mm/sec, a ratio of reduction in sectionof 6:1, according to a sixth embodiment;

FIG. 8 is a microstructure photograph showing a wrought product formedby rolling a magnesium alloy under a roll temperature of 100° C. with asingle reduction in thickness of 40% at a temperature of a test piece of400° C., according to a sixth embodiment;

FIG. 9 is a microstructure photograph showing a wrought product obtainedby performing hot extrusion on a magnesium alloy at a temperature of450° C., an extrusion speed of 2 mm/sec, a ratio of reduction in sectionof 6:1, according to a seventh embodiment; and

FIG. 10 is a microstructure photograph showing a wrought product formedby rolling a magnesium alloy under a roll temperature of 100° C. with asingle reduction in thickness of 40% at a temperature of a test piece of400° C., according to a seventh embodiment.

BEST MODE

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the attached drawings.

A magnesium alloy with a misch metal according to the present inventionhas the formula expressed by Mg_(100-x-y-z)A_(x)B_(y)C_(z), where A iszinc (Zn) or aluminum (Al); B is the misch metal; C is at least oneelement selected from the group consisting of manganese (Mn), nickel(Ni), copper (Cu), tin (Sn), yttrium (Y), phosphor (P), silver (Ag), andstrontium (Sr); and x, y and z are the compositions of 0 at %≦x≦6 at %,0.8 at %≦y≦7 at %, and 0 at %≦z≦2 at %, respectively.

When the magnesium alloy with the misch metal according to the presentinvention is solidified by casting, a magnsium-rich solid solution(alpha magnesium) constitutes a matrix structure. Secondary phases arecrystallized by the misch metal (element B), and form a networkstructure or a dispersed phase that is compositely constructed with amagnesium matrix. This network structure is stable at a hightemperature, and thus provides excellent machanical properties. Further,tertiary phases can be created by element A and C groups, and mainlystrengthen solid-solution/precipitation of the magnesium matrix or thenetwork structure, thereby improving the mechanical properties.

In the case in which aluminum (Al) of the element A group is added alongwith element B in excess of 5 at %, a Mg₁₇A1 ₁₂ phase is formed in thealpha magnesium matrix, thereby deteriorating the mechanical properties.

Thus, the added aluminum is preferably restricted to 5 at % or less.

Further, zinc (Zn) has a peak solid-solution limit of 2.4% at 340° C.with respect to magnesium (Mg). However, considering an amount dissolvedin the secondary or tertiary phase, an addition range of the element Agroup is preferably restricted to 5 at % or less.

In the magnesium alloy with the misch metal according to the presentinvention, aluminum (Al) and zinc (Zn) of the element A group havingsolubility with respect to magnesium are contained in Mg-misch metals,so that multiphases can be obtained. Here, the added misch metal iscomposed of elements having atomic numbers 57 through 71, and includes adidymium-based misch metal or a cerium-based misch metal. Thedidymium-based misch metal is a rare earth alloy composition includingneodymium (Nd) and praseodymium (Pr). Especially, the cerium-based mischmetal refers to a commercialized misch metal alloy which has a maincomposition of 45 wt %≦Ce≦65 wt %, 20 wt %≦La≦30 wt %, 5 wt %≦Nd≦15 wt%, and 0 wt %≦Pr≦10 wt %, and in which other 15 or more trace elementsare present in view of a characteristic in which the misch metal iscrystallized. This misch metal (element B) is caused to form a networkstructure or a dispersed phase which is stable at a high temperature,and improve corrosion and fluidity of the molten metal.

When an addition range of the misch metal (element B) exceeds 7 at %,this is not favorable because a fraction of the secondary phase causingbrittleness is increased, so that the elongation of the material isremoved at a room temperature. Thus, in the present invention, theaddition range of the misch metal (element B) is restricted to 7 at % orless.

In the magnesium alloy with the misch metal according to the presentinvention, in order to promote solid-solution strengthening orprecipitation strengthening or strengthen the network structure in themagnesium matrix structure, an element C group (Si, P, B, Mn, Sr, Y, Ni,Cu, Sn, and Ag) is added. At this time, the added element C group has astrong affinity with magnesium (Mg) or the misch metal. When the elementC group is added at a small amount, it can improve the mechanicalproperties while maintaining the network structure. Exemplary examplesof the element C group are phosphor (P), boron (B), manganese (Mn),strontium (Sr), yttrium (Y), nickel (Ni), copper (Cu), tin (Sn), andsilver (Ag). Accordingly, an addition range of the element C group isrestricted to 2 at % or less so as to be able to expect effects causedby the precipitation/solid-solution strengthening of the matrixstructure while maintaining or strengthening the stable networkstructure at a high temperature.

Further, in the magnesium alloy with the misch metal according to thepresent invention, a small amount of calcium (Ca) is added, so that themagnesium alloy composition can be fused and cast in air without using ashielding gas or flux. An addition range of calcium is restricted to 2at % or less, so as to be able to provide favorable effects of calcium(Ca).

MODE FOR INVENTION

Hereinafter, a magnesium alloy with a misch metal according to exemplaryembodiments of the present invention will be described in detail.

Embodiment 1

A molten metal of a magnesium alloy composition as given in thefollowing Table 1 was prepared, and a cast was obtained by casting. Morespecifically, a carbon crucible was heated in an electric inductionfurnace at a temperature of 700° C. Magnesium was fused in the carboncrucible, and then other addictives were added. Thereby, a molten alloywas formed and poured into a mold, which was pre-heated up to 1200° C.Thereby, the cast was formed.

In the composition specified in Table 1, B refers to at % of acerium-based misch metal. A secondary phase generated by addition ofelement B is a Mg₁₂Ce phase. FIG. 1 is a scanning electron microscopephotograph of Alloy 3, and shows that an alpha magnesium structure andthe Mg₁₂Ce phase form a network structure. Because the structuresforming the network structure were stable at a high temperature and thusinhibited deformation of the alpha magnesium structure, they exhibited ahigh strength at a high temperature. Thus, as an amount of element Bincreased, the Mg₁₂Ce phase increased as well, and both a yield strengthand a tensile strength increased at room and high temperatures. Further,in the case of Alloy 10, the Mg₁₂Ce phase, as the secondary phase, aswell as a tertiary phase were crystallized in the form of an aluminumcompound, and thereby mechanical properties were improved.

TABLE 1 Composition Strength Room E_(corr) (at %) (MPa) Temperature 150°C. 200° C. 250° C. 300° C. (V) Comparative Example AZ91D-T6 YieldStrength 135 100 80 65 50 −1.610 Tensile 280 195 125 98 Strength AE42Yield Strength 113 100 85 Tensile 194 141 100 Strength Embodiment Alloy1 Yield Strength 140 120 107 103 92 Tensile 297 257 197 196 StrengthAlloy 2 Yield Strength 236 177 171 153 133 −1.600 Tensile 355 302 275250 Strength Alloy 3 Yield Strength 192 169 140 127 123 Tensile 352 266265 228 Strength Alloy 4 Yield Strength 280 249 240 204 168 Tensile 426370 362 309 Strength Alloy 5 Yield Strength 265 192 185 149 125 −1.590Tensile 384 297 291 228 Strength Alloy 6 Yield Strength 284 199 194 191132 Tensile 428 355 304 248 Strength Alloy 7 Yield Strength 310 237 215210 174 −1.552 Tensile 443 372 368 283 Strength Alloy 8 Yield Strength387 242 236 223 209 Tensile 518 422 392 320 Strength Alloy 9 YieldStrength 400 350 320 290 219 −1.537 Tensile 537 456 425 358 StrengthAlloy 10 Yield Strength 259 192 174 152 130 −1.556 Tensile 404 298 270217 Strength Alloy 11 Yield Strength 447 397 381 360 315 Tensile 541 526467 414 Strength Alloy 12 Yield Strength 623 574 462 453 445 Tensile 667639 503 483 Strength Alloy 1: Mg_(97.5)Zn₁B_(1.5), Alloy 2: Mg₉₇Zn₁B₂,Alloy 3: Mg_(96.5)Zn₁B_(2.5), Alloy 4: Mg_(95.5)Zn_(1.5)B₃, Alloy 5:Mg₉₆Zn₂B₂, Alloy 6: Mg_(95.5)Zn₂B_(2.5), Alloy 7: Mg₉₅Zn₂B₃, Alloy 8:Mg_(94.5)Zn₂B_(3.5), Alloy 9: Mg₉₄Zn₂B₄, Alloy 10: Mg₉₄Zn₂B₄, Alloy 11:Mg_(92.5)Zn_(2.5)B₅, Alloy 12: Mg_(89.5)Zn_(3.5)B₇.

Accordingly, compared to existing magnesium heat-resistant alloys, themagnesium alloys with the misch metal according to the present inventionwere capable of replacing heat-treatment type sand castingheat-resistant magnesium alloys that maintained a high strength at atemperature of 300° C. or more, and were mainly used at a temperature of200° C. or more, and magnesium alloys formed by die casting, because thesecondary or tertiary phase network structure in which the change instrength depending on the change in temperature was very small wasformed.

The values of E_(corr) given in Table 1 were obtained through potentialmeasurement of an open circuit in a sodium chloride (NaCl) solution of3.5 wt % for 3 hours. Relative corrosion resistance was checked withrespect to the existing heat-resistant alloy, AZ91, which was acomparison target. As a result, it was found that, as the amount ofelement B increased, the corrosion resistance increased.

Embodiment 2

FIG. 2 is a photograph of a molten metal, in which 2 wt % calcium (Ca)is added to Alloy 9 of Table 1 and fused in air, and shows that amagnesium alloy composition can be fused and cast in air by addingcalcium (Ca) to the magnesium alloy composition (e.g. Alloy 9). As canbe seen from FIG. 2, it could be found that a thick oxide was not formedon a surface of the molten metal when the magnesium alloy compositionwas fused in air.

Embodiment 3

TABLE 2 Composition (at %) Hardness Value (H_(v)) Generation of CrackAlloy 12 165 ◯ Alloy 12 + Ni_(0.3) 169 Δ Alloy 12 + Cu_(0.3) 190 X Alloy12 + Sn_(0.3) 176 X Alloy 12 + Al_(0.3) 185 Δ Alloy 12 + Mn_(0.3) 195 XAlloy 12 + Si_(0.3) 191 X ◯: Generation of the crack Δ: Slightgeneration of the crack X: No generation of the crack

In the magnesium alloy with the misch metal according to the presentinvention, as described, the Mg₁₂Ce phase generated by adding the cerium(Ce)-based misch metal to magnesium (Mg) was an intermetallic compoundand had brittleness. Hence, when the Mgl₂Ce phase had a fraction higherthan a magnesium matrix, the magnesium alloy has a property that anelongation was lowered. Therefore, in the current embodiment, an attemptwas made to improve the property by adding a specific element. InEmbodiment 3, Alloy 12 having the highest fraction of the Mg₁₂Ce phasewas selected from the compositions given in Table 1, and then it wasexamined how much the brittleness of the secondary phase was dependenton the addition element.

The molten metals of the magnesium alloy compositions were prepared asin Table 2, and casts were obtained by casting, and subjected to Vickershardness testing. At this time, the examination was performed while theapplied load of indentation was varied from 100 g to 1000 g. In Table 2,it was shown that a hardness value increased by adding nickel (Ni),copper (Cu), tin (Sn), aluminum (Al), manganese (Mn), or silicon (Si) toAlloy 12.

Further, it was found that, when the hardness testing was performed, adegree to which a micro crack was generated around the indentation of asurface was also reduced or disappeared. In Table 2, the increase of thehardness value, or the variation of the generated degree of the crackwas caused by the addition element strengthening a network structure. Inthis manner, according to the current embodiment of the presentinvention, it can be seen that, when elements, such as a C group ofaddition elements (Si, P, B, Mn, Sr, Y, Ni, Cu, Sn, and Ag), having astrong affinity with magnesium (Mg) or cerium-based misch metal areadded to the magnesium alloy with the misch metal of the presentinvention, the mechanical properties are improved.

Embodiment 4

TABLE 3 Elongation (%) Composition Room (at %) Temperature 150° C. 200°C. (Alloy 0.2 2 4 2)_(99.5)Al_(0.5) (Alloy 2)₉₉Al₁ 0.5 4 6 (Alloy 1.5 710 2)_(98.5)Al_(1.5) (Alloy 6)₉₉Al₁ 0.2 2 5 (Alloy 6)₉₈Al₂ 1 6 11 (Alloy6)₉₇Al₃ 1.8 8 16

Table 3 represents elongations obtained by performing a tensile test onthe alloy compositions (e.g. Alloy 2 and Alloy 6) presented inEmbodiment 1, to which Al is added. As shown in Table 3, it can be seenthat, as a trace of Al is changed, the elongation increases. However,when Al is added in excess of 4 at %, this is not favorable, because thenetwork structure capable of maintaining the high strength at a hightemperature is not maintained, and a Mg₁₇A₁₂ phase is created in themagnesium matrix.

As described above, it can be found that the magnesium alloys with themisch metal according to the present invention are high-temperaturestructural magnesium alloys in which the mechanical properties and thecorrosion resistance are greatly improved, compared to the existingheat-resistant magnesium alloys.

Next, a wrought magnesium alloy produced by the magnesium alloy with themisch metal, according to the present invention, will be described.

In the present invention, a magnesium alloy cast to which the mischmetal of the foregoing composition is added is extruded and rolled, andthereby a wrought product is formed. In general, the magnesium alloycannot ensure formability at a room temperature. Hence, in order toobtain a sound wrought product, the magnesium alloy cast is subjected tohot working, and a worked temperature of the magnesium alloy cast is setto a range capable of ensuring soundness of the magnesium alloy castthrough a test. In the case of the extrusion, the magnesium alloy castwas pre-heated and extruded within a range from 350 to 500° C. Thefollowing extrusion conditions are used: an extrusion ratio:6.5:1, anextrusion die angle:180°, a ram speed:2 cm/min.

This extrusion gives rise to dispersion of a secondary phase andrecrystallization of a matrix structure phase, so that an applicationproduct in which strength and elongation are improved can be obtained.Further, when the magnesium alloy cast is repetitively rolled with apercentage of reduction in thickness of 40% at 400° C., and thereby arolled product having a thickness of 1 mm is obtained, the same effectas in the extrusion can be obtained.

A grain refinement mechanism of the magnesium alloy with the misch metalaccording to the present invention makes use of a phenomenon of dynamicrecrystallization in which a nucleus of a new grain is created in astructure during hot working of the magnesium alloy. In the case of themagnesium alloy in which a great deal of particles are distributed, arecrystallization source is increased, and thus grain refinement isconducted in a considerably efficient way. A magnesium alloy in which avolume fraction of other phases than magnesium of a matrix amounts to arange from 5% to 50% in order to maximize such a characteristic is firstformed by casting, and the internal phases that are present in themagnesium alloy are effectively dispersed in the magnesium matrixthrough either hot extrusion or hot extrusion and hot rolling. Thereby,due to the dispersed phases, the dynamic recrystallization iseffectively generated, and thus the grain refinement is maximized.

More specifically, the wrought magnesium alloy with the misch metalaccording to the present invention is expressed by an ordinary chemicalformula Mg_(100-x-y-z)A_(x)B_(y)C_(z), where A is zinc (Zn) or aluminum(Al), B is the misch metal, C is at least one element selected from thegroup consisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn),yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr), and x, y andz are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at%≦z≦2 at %, respectively. Here, the added misch metal is composed ofelements having atomic numbers 57 through 71, and includes adidymium-based misch metal or a cerium-based misch metal. Thedidymium-based misch metal is a rare earth alloy composition includingneodymium (Nd) and praseodymium (Pr), and particularly the cerium-basedmisch metal refers to a commercialized misch metal alloy which has amain composition of 45 wt %≦Ce≦65 wt %, 20 wt %≦La≦30 wt %, 5 wt %≦Nd≦15wt %, and 0 wt %≦Pr≦10 wt %, and in which other 15 or more traceelements are present in view of a characteristic in which the mischmetal is crystallized.

When at least one of these addition elements, i.e. elements capable ofmaking a eutectic, and particularly Al, Si, Ag, Ca, Ni, Cu, Zn, Y, Sn,La, Ce, Pr, Nd, Ce rich-misch metal, and didymium rich-misch metal, isadded to Mg, the magnesium alloy containing the great deal of phases asdescribed above can be formed by casting. When the magnesium alloy issubjected to hot extrusion, its cast structure is subjected to fracture,and other phases than magnesium are granulated and dispersed. For thisreason, the dynamic recrystallization phenomenon is effectivelygenerated to refine the grain.

This hot extrusion makes it possible to perform additional, effectivefracture and dispersion with respect to particles generated due toimpure elements that are inevitably added during the casting of themagnesium alloy.

Hence, an extruded product of the magnesium alloy can be made morestable.

When the extruded product is subjected to hot rolling, a plate of themagnesium alloy in which a great deal of different phases are present inthe magnesium matrix can be formed. In general, when the cast in which agreat deal of different phases are present in the magnesium alloy issubjected to hot rolling, the phases are fractured to act as cracksources, and thus the cast cannot be rolled. However, in the case of theextruded product in which the phases are granulated and dispersed, itsgrains have already been refined, and furthermore their size isrestricted to a particle size even when the crack is generated. Thus,the size does not exert a great influence on the hot rolling, so thatthe extruded product is easily subjected to the hot rolling. Further,even when the temperature for the hot rolling is increased in order toeffectively perform the hot rolling, the grain growth is inhibited dueto the great deal of distributed particles, and thus the hot rolling iseasily performed.

Subsequently, exemplary embodiments of a method of producing the wroughtmagnesium alloy with the misch metal according to the present inventionwill be described.

In the following embodiments, production of extruded products and platesof magnesium alloys around a eutectic point, or a solid-solution limit,and in a hyper-eutectic or hypo-eutectic area will be described.

Embodiment 5

In the present embodiment, an extruded product and a plate are formedusing a Mg—Ce based misch metal-Zn alloy around a eutectic point.

Mg 93.75%, Ce based mish metal 4.25%, and Zn 2.0% by atomic weight weresubjected to mixing and fusion casting, and thereby formed into a slab.FIG. 3 is a photograph of a cast structure of the Mg—Ce based mischmetal-Zn alloy. In order to effectively distribute these eutectic phasesto obtain a magnesium matrix of fine grains, hot extrusion was performedat a temperature of 450° C., an extrusion speed of 2 mm/sec, a ratio ofreduction in section of 6:1.

FIG. 4 is a microstructure photograph of a hot-extruded product of thepresent embodiment. As observed, no crack exists in an internalstructure, and the grains are very fine, an average size of which isless than 14 μm.

In general, in a particle-free magnesium alloy, this grain size cannotbe obtained through single hot extrusion.

Further, the plate of the magnesium alloy was formed by rolling theextruded product under a roll temperature of 100° C. with a singlereduction in thickness of 40% at a temperature of 400° C. FIG. 5 is amicrostructure photograph of a rolled plate. As observed, there is nocrack, and the grains are very fine, an average size of which is lessthan 8 μm.

The formed plate was subjected to a high-temperature tension test. Aphotograph of test pieces of the tested plate is shown in FIG. 6. Thehigh-temperature tension test was performed with strains 1×10⁻³ S⁻¹,1×10⁻² S⁻¹, 1×10⁻¹ s⁻¹, and 1×10⁻⁰ s⁻¹ at a temperature of 500° C., andthe test pieces exhibited high elongations of 580%, 370%, 340%, and250%, respectively. Accordingly, it could be found that the magnesiumalloy having the great deal of phases around the eutectic point wasstably subjected to the hot extrusion and the hot rolling, and hadexcellent formability due to the grain refinement.

Embodiment 6

The present embodiment relates to production of an extruded product anda plate of a Mg−Ce misch metal of a hypo-eutectic area.

Mg 95.7%, and Ce based mish metal 4.3% by atomic weight were mixed, andthen subjected to casting, hot extrusion, and hot rolling on the samecondition as in Embodiment 5. FIG. 7 is a microstructure photograph ofthis test piece hot-extruded on the condition, wherein there is nocrack, and the grains are very fine, an average size of which is lessthan 15 μm. Further, FIG. 8 is a microstructure photograph of a wroughtmagnesium alloy that is subjected to hot extrusion, wherein there arecreated very fine grains, an average size of which is less than 8 μm,without a crack.

In the present embodiment, it could be found that the magnesium alloyhaving the great deal of phases up to the hypo-eutectic area was stablysubjected to the hot extrusion, the hot rolling, and thus the grainrefinement.

Embodiment 7

The present embodiment relates to production of an extruded product anda plate of a Mg—Ce misch metal-Zn alloy of a hypo-eutectic area.

Mg 97.0%, Ce based mish metal 1.5%, and Zn 1.5% by atomic weight weremixed, and the subjected to casting, hot extrusion, and hot rolling onthe same condition as in Embodiment 5. FIG. 9 is a microstructurephotograph of this test piece hot-extruded on the condition, whereinthere is no crack inside, and an average grain size is less than 20 μm.Further, FIG. 10 is a microstructure photograph of a wrought magnesiumalloy that is subjected to hot extrusion, wherein the grains are veryfine, an average size of which is less than 9 μm. In the presentembodiment, it could be found that the magnesium alloy having the greatdeal of phases up to the hypo-eutectic area was stably subjected to thehot extrusion, and thus the grain refinement.

1. A magnesium alloy with a misch metal, having the formula:Mg_(100-x-y-z)A_(x)B_(y)C_(z), wherein A is zinc (Zn) or aluminum (Al);B is the misch metal; C is at least one element selected from the groupconsisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn),yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and x, y andz are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at%≦z≦2 at %, respectively.
 2. The magnesium alloy according to claim 1,further comprising calcium of 2 at % or less.
 3. The magnesium alloyaccording to claim 1, wherein the misch metal is a didymium-based mischmetal or a cerium-based misch metal.
 4. The magnesium alloy according toclaim 3, wherein the didymium-based misch metal is a rare earth alloycomposition including neodymium (Nd) and praseodymium (Pr).
 5. Themagnesium alloy according to claim 3, wherein the cerium-based mischmetal contains 45 wt %≦Ce≦65 wt %, 20 wt %≦La≦30 wt %, 5 wt %≦Nd≦15 wt%, and 0 wt %≦Pr≦10 wt %.
 6. A method of producing a wrought magnesiumalloy with a misch metal, the method comprising the steps of:fusion-casting a magnesium alloy composition having the formula ofMg_(100-x-y-z)A_(x)B_(y)C_(z), where A is zinc (Zn) or aluminum (Al); Bis the misch metal; C is at least one element selected from the groupconsisting of manganese (Mn), nickel (Ni), copper (Cu), tin (Sn),yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr); and x, y andz are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at %, and 0 at%≦z≦2 at %, respectively; and hot-extruding the cast, and refininggrains through granulation and dispersion of other phases than magnesiumin the cast, and recrystallization of a matrix.
 7. The method accordingto claim 6, further comprising a step of hot-rolling the hot-extrudedproduct to form a plate.
 8. The method according to claim 6, wherein thehot-extruding step is performed under the extrusion conditions of atemperature range from 350° C. to 450° C., and a ratio of reduction insection of 5˜80:1.
 9. The method according to claim 7, the hot-rollingstep is performed under the rolling conditions of a temperature rangefrom 350° C. to 500° C., and a percentage of single reduction inthickness from 25% to 50%.
 10. A wrought magnesium alloy with a mischmetal, produced by the steps of: fusion-casting a composition having theformula of Mg_(100-x-y-z)A_(x)B_(y)C_(z), where A is zinc (Zn) oraluminum (Al); B is the misch metal; C is at least one element selectedfrom the group consisting of manganese (Mn), nickel (Ni), copper (Cu),tin (Sn), yttrium (Y), phosphor (P), silver (Ag), and strontium (Sr);and x, y and z are the compositions of 0 at %≦x≦6 at %, 0.8 at %≦y≦7 at%, and 0 at %≦z≦2 at %, respectively; hot-extruding the cast, andrefining grains through granulation and dispersion of other phases thanmagnesium in the cast, and recrystallization of a matrix; andhot-rolling the hot-extruded product to from a wrought product.
 11. Thewrought magnesium alloy according to claim 10, wherein the other phasesthan magnesium have a size of 20 μm or less.
 12. The wrought magnesiumalloy according to claim 10, wherein the other phases than magnesium arecontained from a solid-solution limit to a eutectic point or ahyper-eutectic area.